1. Conventional Materials for Solid Electrolytic Layer
Electrolytic capacitors comprise an oxide film layer functioning as a dielectric material and an electrode drawn out from the oxide film layer, the oxide film layer being formed on an anode electrode comprising a valve action metal such as tantalum and aluminium and being arranged with micro-pores and etching pits.
Herein, the electrode is drawn out, via an electrolytic layer with electric conductivity, from the oxide film layer. Accordingly, the electrolytic layer serves as a practical cathode in such electrolytic capacitors. For an aluminium electrolytic capacitor, for example, a liquid electrolyte is used as a practical electrode, while the cathode electrode only serves for the electrical connection between the liquid electrolyte and an external terminal.
The electrolytic layer functioning as a practical cathode should essentially be adhesive to the oxide film layer, and be dense and uniform. Specifically, the adhesion inside the micro-pores and etching pits of the anode electrode significantly influences the electrical performance. Conventionally, therefore, numerous electrolytic layers have been proposed.
Solid electrolytic capacitors comprise solid electrolytes with electric conductivity, in place of liquid electrolytes defective of any impedence characteristic in the high-frequency region due to the ion conductivity. Specifically, manganese dioxide and 7, 7, 8, 8-tetracyanoquinodimethane (TCNQ) complex have been known as such solid electrolytes.
More specifically, a solid electrolytic layer comprising manganese dioxide is produced by dipping an anode element comprising sintered tantalum in an aqueous manganese nitrate solution, followed by thermal decomposition at a temperature around 300.degree. C. to 400.degree. C. The oxide film layer in capacitors comprising such solid electrolytic layer is readily damaged during the thermal decomposition of manganese dioxide, so the leakage current is likely to increase; because the specific resistance of manganese dioxide is high, additionally, the resulting impedence characteristic is not sufficiently satisfactory.
Furthermore, the lead wire is damaged at the thermal process. Therefore, a post-process is needed to additionally arrange an outer connecting terminal.
Alternatively, a solid electrolytic capacitor comprising the TCNQ complex as described in Japanese Patent Laid-open No. 58-191414 has been known as one of the aforementioned solid electrolytic capacitors, which is produced by thermally melting the TCNQ complex, and dipping an anode electrode in the resulting melt TCNQ complex or coating the resulting melt TCNQ complex on the anode electrode. The TCNQ complex is highly conductive, with the resultant great effects in terms of frequency characteristic and temperature performance.
Because the melting point of the TCNQ complex and the decomposition point thereof are very close so the melt TCNQ complex is readily converted to an insulating substance under some temperature condition, the temperature control of the complex is tough during the process of capacitor production; additionally because the TCNQ complex per se is defective of thermal resistance. the characteristic properties of the complex are distinctively modified by the soldering heat during the mounting process on a print board.
2. Application of Conducting Polymer
So as to overcome the inconvenience of manganese dioxide and TCNQ complex, furthermore, attempts have been made in recent years about the use of conducting polymers such as polypyrrole as solid electrolytic layer.
Conducting polymers typically including polypyrrole are primarily produced by chemical oxidation polymerization (chemical polymerization) and electrolytic oxidation polymerization (electrolytic polymerization). It has been difficult to produce a dense layer with a large strength by chemical polymerization.
By electrolytic polymerization, alternatively, a voltage should be applied to a subject material on which an oxide film layer is to be formed. Therefore, it is difficult to apply electrolytic polymerization to an anode electrode with an insulating oxide film layer formed on the surface thereof for electrolytic capacitors. Hence, a process has been proposed, comprising preliminarily forming a conductive precoating layer, for example a conducting polymer layer formed by chemical polymerization using an oxidant, on the surface of an oxide film layer, and subsequently forming an electrolytic layer by electrolytic polymerization using the precoating layer as an electrode (Japanese Patent Laid-open 63-173313, Japanese Patent Laid-open 63-158829; manganese dioxide functions as the precoating layer).
However, the process of preliminarily forming the precoating layer is complicated; and by electrolytic polymerization, a solid electrolytic layer is formed, starting from the proximity of an outer electrode arranged on the positive electrode face covered with the oxide film layer for the purpose of polymerization. Accordingly, it has been very difficult to continuously form a conducting polymer film of a uniform thickness over a wide range.
Thus, another attempt has been made to form an electrolytic layer comprising a conducting polymer film, by winding an anode electrode and a cathode electrode, both in foil shapes, while a separator is interposed between these electrodes, to form a so-called wound capacitor element, allowing the capacitor element to be impregnated with a monomer such as pyrrole and an oxidant, to form the conducting polymer film by chemical polymerization alone.
Such wound capacitor element has been known for aluminium electrolytic capacitors. It has been desired to avoid any complicated electrolytic polymerization by supporting the conducting polymer layer with a separator and to enlarge the capacity of the resulting capacitor by using an electrode in a foil shape of a larger surface area.
Both the electrodes and the separator can be supported at a constant fastening strength by using the wound capacitor element, which is indicated to make contribution to the adhesion between both the electrodes and the electrolytic layer.
When the capacitor element is impregnated with a mixture solution of the monomer and an oxidant, the monomer and the oxidant are rapidly polymerized together, so that the resulting solid electrolytic layer is never formed deeply inside the capacitor element. Thus, it has been found that the desired electrical performance can never be yielded.
Then, an attempt has been made to lower the polymerization temperature of the solution during the polymerization reaction, with the resultant more or less great electrical performance. Nevertheless, the resulting pressure resistance is still insufficient, disadvantageously.
Additionally, chemical polymerization at low temperature requires strict temperature control and a complicated apparatus, so that the final product is disadvantageously costly.
3. Poly(ethylenedioxythiophene) of Interest
Alternatively, various conducting polymers have been examined. A technique (Japanese Patent Laid-open 2-15611) focused on poly(ethylenedioxythiophene) (PEDT) at a slow reaction velocity and with excellent adhesion to the oxide film layer of the anode electrode has been reported.
With attention focused on the slow polymerization velocity of poly(ethylenedioxythiophene), the present inventors have submitted an application (Japanese Patent application 8-131374) of an invention to generate poly(ethylenedioxythiophene) inside a capacitor element, comprising winding through the medium of a separator an anode electrode foil and a cathode electrode foil to fabricate a capacitor element, allowing the capacitor element to be impregnated with a mixture solution of a monomer and an oxidant solution, and generating a solid electrolyte poly(ethylenedioxythiophene) by the chemical polymerization of the monomer and the oxidant. The polymerization proceeded slowly.
4. Problems that the Invention is to Solve
A solid electrolytic capacitor produced by allowing a capacitor element to be impregnated with a mixture solution of a monomer and an oxidant by using a separator for use in general electrolytic capacitors to generate poly(ethylenedioxythiophene), never exerts satisfactory ESR performance; and additionally, the static capacity and life of the resulting solid electrolytic capacitor are at large variations. This is possibly due to the facts that the use of such general separators is inconvenient for the generation of poly(ethylenedioxythiophene) and that the conditions for allowing the capacitor element to be impregnated with a monomer and an oxidant are not satisfactory. The finding is now described in more detail below.
Because an oxidant ferric p-toluenesulfonate is used for the generation of poly(ethylenedioxythiophene), separators composed of manila paper for use in general electrolytic capacitors induce a chemical reaction, damaging the oxidative action of the oxidant and additionally causing an accident such as short circuit due to the separator damage.
On contrast, glass paper and the like are potentially useable for the separator, but glass paper of general thickness of 80 to 200 .mu.m is hardly slimmed approximately to the thickness of manila paper separator of 40 .mu.m; and because the folding strength is more or less small, a small-size product is hardly produced. Because glass paper is not hydrophilic, a conductive dense and uniform polymer layer, namely solid electrolytic layer, is hardly formed, which possibly affects the electrical performance of the resulting capacitor, disadvantageously.
Additionally, simple impregnation with a mixture solution of a monomer and an oxidant solution does not yield a polymer at a satisfactory polymerization degree, so that a sufficiently dense and uniform solid electrolytic layer is hardly formed inside the resulting capacitor element. During the impregnation with a mixture solution of a monomer and an oxidant solution, in particular, the polymerization reaction of the mixture solution progresses over time, so that the capacitor element is impregnated with the mixture solution, in the course of the polymerization reaction. Thus, the mixture solution is solidified intermediately on the way of the permeation of the mixture solution inside the capacitor element, whereby the resulting solid electrolytic layer is likely to be non-uniform. So as to permeate the mixture solution further inside the capacitor element in order to compensate such intermediate solidification of the mixture solution, the capacitor element should continuously be impregnated with the mixture solution. However, such continuous impregnation of the mixture solution costs needless materials and a longer time, with the resultant decrease of the productivity.
5. Objects of the Invention
The present invention has been proposed so as to overcome the problems. An object resides in the production of a solid electrolytic layer comprising a dense and uniform conducting polymer inside a wound capacitor element, by modifying the separator for use in the capacitor element and the impregnation conditions of the capacitor element with a monomer and an oxidant, to provide a solid electrolytic capacitor with excellent electrical performance and a large capacity. Additionally, the other object is to provide a process for producing such great solid electrolytic capacitor at a high efficiency and a high productivity.